Cellular mobile radio systems are well known. Such systems use a plurality of base stations for broadcasting signals to and maintaining voice and or data communications with mobile radio equipment. Such mobile radios may include automobile carried cellular mobile telephones, portable cellular telephones, pagers, dispatch, and/or data link radios, and a variety of other RF broadcasting equipment that are remote from a central system. Most such radios, which in essence are actually transceivers in that they are typically enabled to both broadcast and receive, are individually identified by a code or call number. Once turned on, each radio transmits an identifying signal that includes the code so that the radio may be contacted if a call is directed to it. That is, the user of the radio does not have to be actively engaged in a communication on the radio for an identifying signal of that radio to be received by the base stations.
Cellular telephony and its related communications technologies has provided numerous benefits to users, including the ability to maintain contact with business associates, friends, and family, and the often critical ability to provide communications to request help and support in an emergency.
Over time, it has become increasingly apparent that the ability to locate the source of a cellular telephone call would be advantageous for a variety of reasons. Of paramount importance is the ability to locate an injured person or law enforcement officer in distress who has initiated a cellular call. Other advantageous reasons relate to locating the original source of an illegally operated cellular telephone.
A disadvantage of conventional systems is that that many are not passive in operation. In other words, some action by the user of the mobile radio may be needed to initiate or maintain tracking by a base station. For example, the user may have to initiate a special signal that may be detected and tracked. It is desirable that locating systems available to law enforcement not rely on initiating action by a mobile radio user.
Known techniques of locating a source of cellular telephone transmission are very similar to efforts directed to radio direction finding methods dating to the First World War. These techniques include triangulation methods where three or more receiving sites are enabled to receive a transmission and correspondingly triangulate a bearing based on the received signals.
Such methods lend themselves well to simple analog radio broadcasts, where a temporal lobe of broadcast power can be homed in upon. However, in cellular telephony, the science of providing the ability of packing more and more users into a finite radio frequency spectrum has created many roadblocks for locating one cellular telephone user out of the potentially thousands of users transmitting in an area at any one time. The rapidly advancing science directed to digital communications compounds matters still further.
For example, in accordance with a frequency division multiple access (FDMA) scheme, a spectral bandwidth of 1.2 MHz can be divided into 120 radio channels with a channel bandwidth of 10 kHz. In accordance with a time division multiple access (TDMA) scheme, a spectral bandwidth of 1.2 MHz can be divided into 40 radio channels with a radio channel bandwidth of 30 kHz, but with each radio channel carrying three time slots for a total of 120 time-slot channels. In a code division multiple access (CDMA) scheme, a spectral bandwidth of 1.2 MHz can be used as one radio channel but can provide 40 code-sequence traffic channels for each sector of a cell. In this scheme a cell of three sectors will have a total of 120 traffic channels. See, for example, Electrical Engineering Handbook, CRC Press, 1993, pages 1552-1553.
One can visualize that as far as channel efficiency is concerned, TDMA, FDMA, and CDMA can be used to provide the same number of traffic channels. However, in FDMA and TDMA, frequency reuse has to be applied.
Unlike FDMA and TDMA signaling systems where there is high likelihood that a mobile unit signal can be received from multiple dispersed sites, such as dispersed base stations, allowing triangulation through lines of bearing or TDOA hyperbolas, CDMA signal levels and co-channel cellular radio characteristics all require other geolocation techniques to guarantee high availability and accurate geolocation.
Further, the conventional triangulation techniques used as such in FDMA and TDMA systems for geolocating a cellular user cannot be adapted to reliably work for systems utilizing emerging CDMA air standards such as IS-95. See, Mobile Radio Communications, Steele, R., IEEE Press, 1992, pages 71-76, herein incorporated by reference.
Unlike the air standards applicable to TDMA and FDMA systems, IS-95 allows multiple users to share common frequency spectrum simultaneously, by assigning each system user a unique spread spectrum spreading code. In spread spectrum a modulation technique is used for multiple cellular subscriber access, or for increasing immunity to noise and interference.
A spread spectrum system makes use of a sequential noise-like signal structure, for example P.N. (pseudo-noise) codes, to spread the normally narrow band information signal over a relatively wide band of frequencies. The receiver correlates these signals to retrieve the original information signal. Foundations of Mobile Radio Engineering, Yacoub, CRC Press, 1993, pp. 401-7, here incorporated by reference.
Because a CDMA base station field of view might include thirty or more users not separable in time or frequency, another approach must be taken for reliably distinguishing each user for geolocation. Further, because IS-95 employs a frequency reuse factor of one, geolocation of mobiles must routinely be done from a single site rather than three, potentially adding additional complexity.
Because having an accurate time standard in relationship to the coded communications is of paramount importance to the integrity of a CDMA system, in at least one known system, a geo-synchronous satellite based time standard is utilized in conjunction with a broadcast pilot signal. Steele, p.71.
Accordingly, it would be advantageous to provide an method and system for determining the position of a mobile radio without requiring a multiplicity of receiving sites.
It would also be advantageous to provide a means for geo-locating the position of a spread spectrum radio frequency emission, such as a CDMA coded radio frequency emission.
It would also be very advantageous to provide an apparatus and method for locating the position of a CDMA cellular coded radio frequency emission in essentially real time.